1. Field of the Invention
The invention relates to tunable filters for optical networks.
2. Discussion of the Related Art
An important element of wavelength-division-multiplexed (WDM) optical networks is a wavelength-selective optical switch, commonly referred to as an add/drop filter. (WDM systems are discussed, for example, in D. Al-Salameh et al., "Optical Networking," Bell Labs Technical Journal, Vol. 3, No. 1, January-March 1998, at 39, the disclosure of which is hereby incorporated by reference.) The add/drop filter allows addition or removal of a selected wavelength to or from the multiple-wavelength WDM signal in an optical fiber. Currently, two types of add/drop filters are used commercially--integrated lithium niobate filters and fiber gratings. The integrated lithium niobate filter typically uses (a) a Dragone arrayed-wavelength-grating (AWG) (as shown in FIG. 4 of Salameh et al., supra) to separate the input channels, i.e., individual wavelength spectra separated by a wavelength spacing, from the WDM signal into individual fibers, (b) lithium niobate electro-optic switches on individual fibers to perform add/drop on each channel, and (c) another Dragone AWG to recombine the wavelengths into a WDM signal on a single fiber. (See, e.g., C. Dragone et al., IEEE Photonic Technology Letters, Vol. 3, 1991, at 896.) This apparatus provides useful results, but is relatively costly and occupies an undesirably large amount of space.
Fiber gratings are permanent, periodic variations in a fiber's index of refraction that are introduced during manufacture. (See, e.g., T. Erdogan and V. Mizrahi, IEEE LEOS Newsletter, February 1993, at 14.) Particular wavelength regions of light traveling along the fiber are reversed in direction by the grating (the light is reflected back in the direction from which the light originated) if the well-known Bragg conditions for reflection are met, i.e., where the optical wavelength is equal to twice the grating period. This reversal of direction is desirable due to its relative simplicity: (a) the light either passes through the grating (for wavelengths where the Bragg condition is not met) or is reversed (for wavelengths where the Bragg condition is met), and (b) the input signals and output signals are, respectively, kept together for further distribution. Unfortunately, fiber gratings generally are not able to be reconfigured in real-time.
Efforts to improve on integrated lithium niobate filters and fiber gratings have involved filters which utilize acoustics to provide real-time tunability to select desired wavelength channels. For example, as discussed in C. Tsai et al., Appl. Phys. Lett., Vol. 26, 1975, at 140, and reflected in FIG. 1, it is possible to use a surface acoustic wave (SAW) to interact with the optical beam in a planar optical wave guide 10 located on a lithium niobate substrate 12. The SAW is generated by a high frequency voltage applied to the interdigital SAW transducer 14 (IDT) (partially shown) located on the waveguide 12. (Transducers useful in SAW devices are discussed, for example, in R. Rosenberg, "Surface Acoustic Wave Filters," in Miniaturized and Integrated Filters, 329 (S. Mitra and C. Kurth, eds., 1989), the disclosure of which is hereby incorporated by reference.) Non-normal input optical beam 16 having wavevector k(op).sup.in enters the waveguide at an angle .theta. with respect to an acoustic wave 18 having wavevector k(ac). The beam 16 is reflected as light 17 having a wavevector k(op).sup.out if the acoustic and optical wavevectors satisfy the well-known Bragg conditions for reflection. ("Op" indicates optical wave and "ac" indicates acoustic wave.) Input optical waves at other wavelengths will pass through the optical waveguide 10 as unscattered light 19. Unfortunately, the lateral spreading of the acoustic wave and the non-normal incidence of the input beam 16 is undesirable in some circumstances. In addition, because the optical input is at an angle .theta..noteq.0, birefringence of the incident ray will generally occur, although it is possible to reduce the detrimental effect of such birefringence by polarizing the input beam.
Another approach utilizing acoustic waves is the mode-conversion acousto-optic tunable filter (AOTF), as discussed, for example, in U.S. Pat. No. 5,652,809 to Aronson and U.S. Pat. No. 5,611,004 to Chang et al., the disclosures of which are hereby incorporated by reference. As shown in FIG. 2, a mode-conversion AOTF is fabricated in an elongated crystalline piezoelectric substrate 20. The substrate 20 is typically lithium niobate, oriented (as shown in FIG. 2) with its x-z plane perpendicular to the direction of the input light to provide a desired propagation of the input light. An optical waveguide 22 is formed on an upper surface of the substrate 20, typically by interdiffusion of titanium. Input light 24, which is passed through a polarizer 26 to provide a single polarization mode, propagates through the waveguide 22, and an acoustic wave is introduced by applying an electrical signal to an interdigital transducer 28. The acoustic wave induces a refractive index grating, such that the transverse electric (TE) and transverse magnetic (TM) polarization modes are coupled for a particular band of optical wavelengths, and within this particular band, light is converted to a polarization mode orthogonal to its previous polarization mode. An output polarizer 30, at a right angle to the input polarizer, thus blocks all but these selected optical wavelengths. Such mode-conversion AOTFs offer useful results, but the unselected wavelengths are lost unless a polarizing beam-splitter is used. In addition, the acoustic waves are not maintained in the optical waveguide, and thus lose their effectiveness as they propagate through the material. Moreover, the frequency response of mode-conversion AOTFs is often unacceptable due to the inclusion of large sidelobes in the frequency response. Sidelobes are peaks in the response at frequencies falling outside the selected optical wavelength channels, and tend to result in the selection of a portion of undesired wavelength channels. Such sidelobes thereby detrimentally affect the usefulness and precision of mode-conversion AOTFs.
Thus, while useful tunable add/drop filters have been developed, filters offering improved properties are desired.